As described earlier (see Stack Layout), Guile’s calling convention is that arguments are passed and values returned on the stack.
For calls, both in tail position and in non-tail position, we require
that the procedure and the arguments already be shuffled into place
befor the call instruction. “Into place” for a tail call means that
the procedure should be in slot 0, and the arguments should follow. For
a non-tail call, if the procedure is in slot n, the arguments
should follow from slot n+1, and there should be two free slots at
n-1 and n-2 in which to save the
Returning values is similar. Multiple-value returns should have values
already shuffled down to start from slot 1 before emitting
return-values. There is a short-cut in the single-value case, in
return handles the trivial shuffling itself. We start from
slot 1 instead of slot 0 to make tail calls to
In both calls and returns, the
sp is used to indicate to the
callee or caller the number of arguments or return values, respectively.
After receiving return values, it is the caller’s responsibility to
restore the frame by resetting the
sp to its former value.
Call a procedure. proc is the local corresponding to a procedure. The two values below proc will be overwritten by the saved call frame data. The new frame will have space for nlocals locals: one for the procedure, and the rest for the arguments which should already have been pushed on.
When the call returns, execution proceeds with the next instruction.
There may be any number of values on the return stack; the precise
number can be had by subtracting the address of proc from the
Tail-call a procedure. Requires that the procedure and all of the arguments have already been shuffled into position. Will reset the frame to nlocals.
Tail-call a procedure. The procedure should already be set to slot 0.
The rest of the args are taken from the frame, starting at from,
shuffled down to start at slot 0. This is part of the implementation of
Receive a single return value from a call whose procedure was in proc, asserting that the call actually returned at least one value. Afterwards, resets the frame to nlocals locals.
Receive a return of multiple values from a call whose procedure was in
proc. If fewer than nvalues values were returned, signal an
error. Unless allow-extra? is true, require that the number of
return values equals nvalues exactly. After
has run, the values can be copied down via
mov, or used in place.
Return a value.
Return a number of values from a call frame. This opcode corresponds to
an application of
values in tail position. As with tail calls,
we expect that the values have already been shuffled down to a
contiguous array starting at slot 1. We also expect the frame has
already been reset.
Capture the current continuation, and tail-apply the procedure in local
slot 1 to it. This instruction is part of the implementation of
call/cc, and is not generated by the compiler.